In many polymer processes the polymerization reaction is carried out in a vehicle which is a solvent for both the monomers to be polymerized and the polymer formed. In such solvent polymerization processes, the separation of the polymer from the vehicle is generally an energy intensive step where the separation is usually carried out by steam stripping or other suitable solvent evaporation techniques. It has long been recognized that substantial economies in polymer processes could be achieved if the energy requirements of the solvent-polymer separation step could be minimized.
It is well known that many solvent-polymer solutions are stable over a limited temperature range and can be caused to separate into a solvent rich and polymer rich phase by heating or cooling. Upon heating, these solutions exhibit a lower critical solution temperature (LCST) above which separation of the polymer from the solvent system will occur. This separation results in the formation of two distinct phases, one a solvent rich phase, the other a polymer rich phase. These phase separation phenomena are generally pressure dependent, and the two phase systems can be made to revert to a homogeneous single phase by isothermally increasing the pressure of the system above a critical value which depends upon the composition of the solution and the molecular weight of the polymer. The phase behavior of a typical polymer solution is shown schematically in FIG. 1A, as is discussed later.
The LCST is that temperature above which a solution will separate into two distinct phases, a solvent rich phase and a solute rich phase. The separation phenomenon can also occur at a second lower temperature termed the Upper Critical Solution Temperature (UCST). Below the UCST a two phase separation again occurs. The measurement of LCST and UCST end points are made at the vapor pressure of the solution. The prior art teaches a number of methods of utilizing the LCST as a means for causing a polymer solution to separate into a polymer rich phase and a solvent rich phase.
Illustrative prior art processes which have utilized the LCST phenomenon in polymer separation processes are those described in U.S. Pat. Nos. 3,553,156; 3,496,135; and 3,726,843 incorporated herein by reference.
These prior art processes are disadvantageous in that a significant amount of heat energy is required to raise the temperature to the point where the desired phase separation occurs. Furthermore, separation occurs at elevated temperatures which may result in polymer degradation. Separation processes utilizing the UCST are also disadvantageous because of the need to cool the solutions. More recently, in their U.S. Pat. No. 4,319,021, Irani, et al. have taught an improvement in the foregoing phase separation processes which permits the use of lower separation temperatures. The technique described in this patent includes the addition of a low molecular weight hydrocarbon to the polymer solution. Suitable low molecular weight hydrocarbons are the C.sub.2 -C.sub.4 alkenes and alkanes which are utilized at about 2 to about 20 weight percent. While this improved process substantially reduces the phase separation temperature, heating is still required in order to affect the desired separation.
There is need for a process technique which would permit the aforedescribed separation processes to be carried out at or near the polymerization reaction exit temperature. In that way, little or no additional heat input would be required to effect the separation. Heretofore, such idealized, improved processes have not been achieved.